The present invention relates to a scan type anamorphic magnifying apparatus capable of a so-called anamorphic magnification for appropriately enlarging or reducing an image of an original document scanned by a slit exposure method through varying a vertical-lateral proportion of the image, and more particularly to the scan type anamorphic magnifying apparatus comprising a scanning device for scanning the document through a slit in a direction perpendicular to a longitudinal direction of the slit, a photosensitive member shifted at a substantially constant speed in a certain direction, an image forming device for projecting and forming the image of the document scanned by the scanning device on the photosensitive member, speed change means for varying a speed ratio between the scanning speed of the scanning device and the shifting speed of the photosensitive member, and the like whereby the image of the original document is formed, through the working of the speed change means, on the photosensitive member in a proportion different in the scanning direction and the direction perpendicular thereto.
This type of anamorphic magnifying apparatus is utilized, for example, for producing a copy of an original as a printing source in a shape reduced in only one direction in expectation of the elongation of the printing source when it is set on a cylinder of a printing machine or for creating extra space for filing on the copy paper.
More specifically, by setting the magnification through the image forming device to .beta.1 and by setting the scanning speed v of the scanning device relative to the shifting speed V of the photosensitive member to: EQU v=v2 (v2=v/.beta.1.multidot..beta.2)
a copy of the original document magnified by .beta.1.multidot..beta.2 in the scanning direction and by .beta.1 in the direction perpendicular thereto is obtained.
However, since the scanning speed of the image of the document projected on the photosensitive member by the image forming device differs from the shifting speed of the photosensitive member, there may occur a reduction in resolving power due to a slipping in the image of the document formed on the photosensitive member.
This will be more particularly described next with reference to FIG. 10, which schematically shows a state where the image of the document is projected on the photosensitive member.
In this figure, a width of a slit 3 is denoted by d, the magnification through the image forming device I by .beta.1 and the scanning speed v relative to the shifting speed V of the photosensitive member 6 is set to (v=V/.beta.1.multidot..beta.2). An exposure lamp is not shown.
A point A0 on the document M, when crossed by an end of the slit 3, is projected on a point A relative to the photosensitive member 6. The point A0 on the document M, when crossed by the other end of the slit 3 with the movement of the scanning device, is projected on a point A' relative to the photosensitive member 6. That is to say, the image of the point A0 projected on the photosensitive member 6 is shifted from A to A' with the scanning operation. The shifting distance D1 is represented by a length of the slit width d magnified by .beta.1: EQU D1=.beta.1.multidot.d
Whereas, a point B corresponding to the point A and on the photosensitive member 6 shifted at the speed V is shifted to a point B'. This shifting distance DD is derived from the following equation: ##EQU1## Thus, DD differs from the shifting distance D1 of the projected image. This difference .DELTA. between the above two shifting distances D1 and D2: ##EQU2## causes the resolving power reduction in the image formed on the photosensitive member 6.
In order to solve this problem, there is known construction comprising an anamorphic optical element such as a cylindrical lens having a refracting power only in a direction corresponding to the scanning direction and disposed in a projection optical path extending toward the photosensitive member thereby to equate the shifting distance of the image projected on the photosensitive member and the shifting distance of the portion of the image on the photosensitive member corresponding thereto.
However, according to the above-described conventional construction, although the resolving power reduction due to the difference between the scanning speed of the document and the shifting speed of the photosensitive member reduces, there occurs a disagreement between focal planes relative to the scanning direction and to the direction perpendicular thereto. This problem will be more specifically described next with reference to FIGS. 8(a), (b) and FIGS. 9(a), (b).
Referring to FIGS. 8(a) and 8(b), in this case, a cylindrical lens 7c biconvex in section by way of example of the anamorphic optical element having the refracting power in the direction corresponding to the scanning direction, is disposed in the projection optical path extending toward the photosensitive member 6. As shown in FIG. 8(a), the cylindrical lens 7c having a positive refracting power in the slit width direction, i.e. the direction corresponding to the scanning direction ( the vertical direction in the same figure) causes the focal plane FS1 of the projection light from the image forming device I to shift close to a photosensitive member surface 6a by .delta.1.
At the same time, the cylindrical lens 7c, when acting as a parallel flat plane in the longitudinal direction of the slit, causes the focal plane FS2 of the projection light from the image forming device I to shift away from the photosensitive member surface 6a by .delta.2.
If the cylindrical lens 7c is provided with a large refracting power in order to enlarge the vertical to lateral magnification of the formed image, i.e. the anamorphic magnification, these two focal planes FS1 and FS2 are displaced in mutually opposite directions relative to the photosensitive member surface 6a, whereby either of the focal planes, FS1 or FS2, or the both FS1 and FS2 may not be confined within a focal depth. If this happens, the image formed on the photosensitive member 6 is blurred.
In the next case, a cylindrical lens 7d biconcave in section is disposed in the projection optical path extending toward the photosensitive member 6. Referring now to FIG. 9(a), the cylindrical lens 7d having a negative refracting power in the slit width direction, i.e. the direction corresponding to the scanning direction (the vertical direction in the same figure) causes the focal plane FS3 of the projection light from the image forming device I, in contrast to the previous case, to shift away from the photosensitive member surface 6a by .delta.3. At the same time, in the longitudinal direction of the slit, as shown in FIG. 9(b), the lens 7d, acting similarly to the previous case, causes the focal plane FS4 to shift away from the photosensitive surface by .delta.4.
In this case, in contrast to the previous case employing the cylindrical lens 7c having the positive refracting power, the directions of the displacements of the two focal planes FS3 and FS4 agree with each other. Thus, it is possible to reduce the mal-effect caused by the displacements of the focal planes by adjusting the distance between the photosensitive member 6 and the image forming device I as to permit the photosensitive member surface 6a to be disposed in a middle position between the focal planes FS3 and FS4. However, in this case also, if the cylindrical lens 7d has a large refracting power in order to enlarge the anamorphic magnification, the distance between the focal planes FS3 and FS4 may not be confined within the focal depth and the image formed on the photosensitive member 6 is blurred.